@Kevin_Z 

Thanks Kevin for your reply and the link.

I was aware of these instructions and I followed them exactly as described but without success.

I am away from location for a period, and I cannot send screen prints of the router settings for the next days.

Also I am hesitant to upgrade the firmware, as the MR6400 works just fine in 3G/4G mode and reading other users' experiences when upgrading the firmware, I do not want to create any other problem.

I noted that when I connect the WAN port of MR6400 with the fixed line router via an ethernet cable, the DHCP server of the fixed line router gives an IP address to MR6400  of 192.168.1.3, but the MAC address of MR6400 shows changed in the first and last group.  These settings in the LAN list  of devices of the fixed line router show greyed (no active) contrary to black for other active devices.

The Main Gateway address of the fixed line router is 192.168.1.1  Knowing that the MR6400 IP address is also 192.168.1.1 shouldn't this cause a conflict?

Do I have to assign a fixed IP address of the MR6400 to the Main Gateway router (the fixed line) eg. 192.168.1.3 with the correct MAC address?

I am trying to figure out the logic behind the settings in order to solve the problem.

Is the MR6400 in wireless router mode working as an access point? If yes, its DHCP server should be disabled, and should be enabled automatically when fixed line internet fails.

Is this the case?

Many Thanks

@ipourn 

I use this device the same way, have you disabled the DHCP on it?

@Permetris 

I have no access to the 3g router right now as it is installed in a another far away location. However, if I am not mistaken (correct me if I am wrong) the DHCP server is automatically disabled when operation as a wireless router is selected.

@Kevin_Z  please Sir, i am having a serious issue with my mr6400 V4(EU) constant disconnection. 

 here is my current firmware version: 1.16.0 0.9.1 v0001.0 Build 191223 Rel.66638n

Your firmware is up to date.